Waveguide filter utilizing evanescent waveguide, with tunable ferrite loading

ABSTRACT

A waveguide filter wherein transversely magnetized ferriteloading strips are mounted within a waveguide section to produce cutoff at a higher frequency than in an empty waveguide, thereby providing evanescent-mode operation at the operating frequencies. The waveguide section is then terminated in a capacitive reactance which at the center frequency of the desired passband is the conjugate of the positive imaginary characteristic impedance of the length of evanescent waveguide. Means for varying the magnetic field applied to the ferrite-loading strips is provided for tuning the filter over a predetermined range of passband frequencies.

1Jit tates aertt Craven et al.

[ 1 Feb. 1,1972

[54] WAVEGUIDE FKLTER UTIILHZHNG EVANIESCENT WAVEGUIDE, WITH TUNABLEFERRITE LOADING [72] Inventors: George Frederick Craven, Sawbridgeworth;Richard Finnie Slredd, Bishop's Stortiord, both of England [73]Assignee: International Standard Electric Corporation, New York, N.Y.

[22] Filed: Nov. 9, 1967 211 App1.No.: 681,637

[30] Foreign Application Priority Data Nov. 30, 1966 Great Britain..53,635/66 [52] U.S.Cl. ..333/73 W, 333/24.1, 333/81 [51] Int. Cl..H03ll 7/00, H03h 7/10 [58] Field ofSearch ..333/73,73 C, 73 W, 95, 97,

[56] References Cited UNITED STATES PATENTS 3,051,908 8/1962 Anderson etal ..330/5 2,816,270 12/1957 Lewis ....333/9 3,215,955 11/1965 Thomas..333/7 2,866,949 12/1958 Tillotson ..333/1 1 3,237,134 2/1966 Price....333/73 W 3,496,498 2/1970 Kawahashi ..333/73 W OTHER PUBLICATIONS B.Lax and K. .1. Button Microwave Ferrites" Pub. by Mc- Graw-Hill BookCo., N.Y. 1962, pp. 367- 373 QC753L3.

Primary Examiner-Herman Karl Saalbach Assistant Examiner-C. BaraffAttorneyC. Cornell Remsen, Jr., Rayson P. Morris, Percy P. Lantzy,Philip M. Bolton and lsidore Togut [5 7] ABSTRACT A waveguide filterwherein transversely magnetized ferriteloading strips are mounted withina waveguide section to produce cutoff at a higher frequency than in anempty waveguide, thereby providing evanescent-mode operation at theoperating frequencies. The waveguide section is then torminated in acapacitive reactance which at the center frequency of the desiredpassband is the conjugate of the positive imaginary characteristicimpedance of the length of evanescent waveguide. Means for varying themagnetic field applied to the ferrite-loading strips is provided fortuning the filter over a predetermined range of passband frequencies.

9 Claims, 14 Drawing Figures I WAVEGUIDE FILTER UTILIZING EVANESCENTWAVEGUIDE, WITI-I TUNABLE FERRI'IE uosumc BACKGROUND OF THE INVENTIONThe invention relates to waveguide filters.

Waveguide band-pass filters have heretofore been constructed inwaveguide in which a propagating mode-usually the dominant-existsSUMMARY OF THE INVENTION According to the invention there is provided anH-wave band-pass filter or filter section comprising a length ofwaveguide loaded with ferrite material such that when subjected to atransverse unidirectional magnetic field the effective dimensions of thelength of waveguide are such that only evanescent I-I-waves can existtherein at the operating frequency. Further provided is means forterminating the said length of waveguide in a capacitive reactance whichat the center frequency of the desired passband is the conjugate of thepositive imaginary characteristic impedance of the said length ofwaveguide.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows two lengths ofpropagating waveguide interconnected by a three-section band-pass filterembodying the invention,

FIG. 2 shows effective permeability vs. angular frequency fortransversely magnetized ferrite,

FIG. 3 shows a section of waveguide loaded with ferrite sidewall strips,

FIG. 4 shows insertion loss vs. frequency for a ferrite-loaded sectionof waveguide in cutoff condition with DC magnetic field as a parameter,

FIG. 5 is the equivalent circuit of one section of the bandpass filterof FIG. 1,

FIG. 6 is the circuit of FIG. 5 bisected,

FIG. 7 is the image impedance characteristic of the bandpass filter ofthe present invention,

FIG. 8 shows an alternative form of the band-pass filter of FIG. I,

FIG. 9 shows another form of band-pass filter embodying the invention,

FIG. 10 is the approximate equivalent circuit of the bandpass filter ofFIG. 9,

FIG. 11 shows the filter section of FIG. 9 coupled as aseries stub todominant-mode waveguide,

FIG. 12 shows the filter section of FIG. 9 coupled as a shunt stub to adominant-mode waveguide,

FIG. 13 shows a single-section band-pass filter coupled betweendominant-mode waveguides; and

FIG. 14 is the equivalent circuit of the single'seetion bandpass filterof FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I,dominant-mode H-waves are propagated in a length l of propagatingwaveguide from any suitable microwave source (not shown), such as agenerator or a receiving aerial. A length 2 of propagating waveguide isinterconnected with the length l by a three section band-pass filter 3constructed in the same waveguide as the lengths 1 and 2, but containingloading strips 4 of ferrite material symmetrically arranged one at eachsidewall of the waveguide. There are three capacitive screws 5, and theferrite material strips 4 are subjected to a DC magnetic field Htransverse to the direction of propagation by suitably positionedpermanent or electromagnetic pole pieces.

In FIG. I there is shown an electromagnetic apparatus for producing theDC magnetic field which includes a core having a winding 21woundthereon. Coupled to winding-2l is a source of energy 22, the outputof which is variable. This enable the magnitude of the magnetic field Hto be varied. The production of the DC magnetic field by means of apermanent magnet 23 is shown in FIG. 9.

It has been established 'that for a transversely magnetized ferrite inrectangular waveguide (H mode) the cutoff frequency may be controlled bythe DC magnetic field. The cutoff frequency can be made higher or lowerthan the empty '5 waveguide value. This is a consequence of the factthat the effective permeability 11. of the ferrite can be varied frompositive to negative values by the DC magnetic field as shown in FIG. 2,in which ne is the cutoff frequency and w, gyromagnetI'c resonance forthe infinite ferrite medium. Thus for a 0 the RF field is concentratedin the ferrite and the effective width of the waveguide is increased,whereas for p 0 the RF energy is excluded from the ferrite and theeffective width is reduced. It is this latter effect which is used inthe present in vention, and is illustrated in FIG. 3.

15 In a rectangular waveguide loaded with ferrite magnetized transverseto the direction of propagation as shown in FIG. 2, the transcendentalequation involving the required propagation constant B, by solving theboundary value problem for such a structure, is

2 Kipi sin Kmfi cos KmO l e (I tan k, (L28) B 0 (Kmflo) 1 sm-Km8 cos KmoI pOKa l m u j 0 11 jk t 0 =tensor permeability of the ferrite 30 P: am1+X m (FULICZ) l e rr) u L 1 X H TJ'I u U (WE/M1 5. B- 4) JiF/e Y4-7TMYH JZJ' 1 [(YHi) ---w-] 'jwY4'wM, I k i)* 411M, saturation magnetizationof the ferrite Y= gyromagnetic ratio H, internal DC magnetic field inferrite k,, propagation constant in the ferrite in x direction k,,propagation constant in air in x direction e permittivity of the ferriteL= width of rectangular waveguide 8 thickness of ferrite material Withinthe ferrite the RF fields vary as i(k,,.rl3y) and in the air part asJ(k,,xl3y) From equation 1 the condition for cutoff, [#0 yields theexpression If by some appropriate means it can be arranged that km =jlki.e. the propagation constant in the x direction in the ferrite is real(corresponding to an exponential behavior with distance x) then equation4 may be written w pt e and it follows that by having than I /\'m I thenB is made negative i.e., evanescent condition. This is brought about byadjustment of the DC magnetic field on the ferrite (by varying theoutput of source 22 of FIG. I, for example) so that for the frequency ofoperation p is made sufficiently negative (see FIG. 2).

The net effect is as if the width L of the waveguide has been reduced.

The width L of the waveguide in FIG. 1 is in the evanescent conditionbrought about as explained above by the DC magnetic field I-I Thiscondition is illustrated in FIG. 4 and FIG. 4 also shows how the cutofffrequency of the section is dependent on the value of DC magnetic field.The field, and how the insertion loss increases at a specific frequencyfor increases in the strength of the DC magnetic field may be madevariable in value when applied as shown in FIG I by varying the outputof the energy source 22. When permanent magnet means is used to providethe electromagnetic field I-I the field may be varied by changing theposition of the magnetic poles with respect to the waveguide structure.

The filter is tunable in frequency by variation in the value of the DCmagnetic field. An increase in field raises the frequency, and areduction in field lowers the frequency.

Waveguide which is evanescent has a positive imaginary characteristicimpedance i.e., at its input tenninals (if infinitely long) it willappear as a pure inductance. Consider now a section of the filter ofFIG. I of finite length I terminated in a capacitance C,, and apropagation constant 3 Such a section is shown in the familiar T-sectionform in FIG. 5. Because the network is symmetrical it can be bisected(FIG. 6) and its properties detemu'ned in terms of its open-circuit andshort circuit parameters. The A matrices for such a network are;

E, cos h 11 ,2, sin h ll 1 E2 1 1. 7 sin h cos h ,B, 1 12 The combinedmatrix is then The image impedance is th e n given by r ZMZM- l l j slnI1 2 +1131 COS h 2 The image transfer constant cos I1 is given by cos Ird =2 (cos /r'- Lj-ZJI. sin I: cos I:

- b (I la) Standard texts on image parameter filter theory state thatthe band limits occur when This is so in (1 la) if either I I I cos h" 7-2 3 sin Ir cos I1 Then Alternatively, if

l cosh In equations (14) and (15) the susceptance is given by the usualexpression I tan h :7

The center frequency,f,,, occurs at the geometric mean,

ffrf:

Therefore,

Further. 5

center frequency, fi,. Substituting the value of B at the m centerfrequency 21rfi C in the image impedance,

10, fll IS given by An obvious characteristic of the filter is that itsbandwidth is a function of y and (in the ideal lossless case) as yl mthen tanh yl v cot/t yl and the bandwidth (I'm/' reduces towards zero.The behavior of the image impedance as a function of frequency isinteresting. At very low frequencies 71 is extreme ly large and B, tendsto zero. Thus Z, is given by Z =jZ,,. At the lower frequency band limit,f,, the denominator in (IQ) becomes zero so that Z, becomes infinite. Atthe upper frequency limit, f the numerator becomes zero and, there fore,Z,- is also zero. The behavior is illustrated in FIG. 7.

Returning to FIG. I, the rejection below resonance is intrinsicallyhigher than other filters (because y increases with wavelength) and theloss per sections is not excessive. As in all microwave filters unwantedpassbands can occur but the usual dominant-mode multipleicavityresonance effect is obviously absent. In the simplest case whenidentical guide is used throughout free transmission must be expectedabove the eutoff frequency for that particular guide. In order tocontrol this efiect the construction of FIG. 8 is used where thecapacitive screws are replaced by capacitive ridges 6. This style ofconstruction is, of course, identical to that of the familiar corrugatedlow-pass filter. It is then possible to completely suppress thispassband, or alternatively, employ it as a second controllable passbandin systems requiring this feature. The magnetic field I-I,, may beapplied as shown in FIGS. 1 or 9.

Networks of the type described above have useful properties as reactancenetworks when considered as purely singleport devices. This isillustrated by the network of FIG. 6 with its output terminals opencircuited. The input impedance is then given by (1011) which slightlyrearranged is rearranged is Z=-jZa m (B, cosh sinh This network has thezero and infinity values of 2, as described above and is roughly theequivalent of the m-derived section shown in FIG. 9. The approximateequivalent circuit is given in FIG. 10.

In this form of filter, the length of evanescent waveguide 10 may beterminated by a short-circuited section of propagating waveguide 11having a length lsuch that tan (21rI/Ag) is negative, and thus forms therequired terminating capacitive reactance for the length of evanescentwaveguide. This form of terminating prevents energy being lost at thetermination if this form of construction is used for example as a stub.A permanent magnet 23 supplies the magnetic field H It is pointed outthat magnets having other shapes and movable poles may be used.

Several applications for the reactance section of FIG. 9 are now given.When coupled to conventional dominant-mode guide 12 it can be used as ashunt or series stub in the usual way ([3 or I4 in FIGS. 11 and 12respectively). By using it as a shunt stub the passband appears at alower frequency than the rejection band; as a series stub the positionsof the two bands are reversed. It can, for instance, be used as theseries element in a terminating m-derived section for the evanescentwaveguide filter. Alternatively, in this type of filter it can be usedas part of an internal section giving high rejection at a specifiedfrequency.

A more accurate version of the equivalent circuit of a single-sectionfilter similar to that of FIG. 1 is shown in FIG. 14, the filter 15being shown schematically in FIG. 13 as a length of evanescent waveguide16 with a central capacitive screw 17, between dominant-mode guides 18and 19.

The inclusion of the inductance shunt susceptances, representedby thejunction with dominant-mode guide, has the effect of bringing the tworesonances much closer together than predicted by The junctionsusceptances, if sufficiently large can completely eliminate theresonances. Experiments with X-band guide at 4,000 mc./sec. (junctionbetween 2X2/3-in. and 0.9X0.4-in. guides) failed to demonstrate thiseffect until'the junction susceptances were tuned out with capacitivescrews in shunt.

By constructing the filter to have capacitive screws at each end oltheevanescent waveguide section to have the form ol'a 11' section, thecapacitive screws then serve both to tune out the junction susceptancesand as the terminating capacitive reactance of the filter section.

The evanescent waveguide section or sections may be constructed inwaveguide of different dimensions from that of the propagatingwaveguide. If the waveguide dimensions are larger than that required forthe cutoff condition at the operating frequency, the ferrite materialloading strips have applied thereto a transverse DC magnetic field suchthat the effective permeability of the material is made sufficientlynegative to bring the section to the evanescent condition.

Conversely, the waveguide dimensions may be smaller than required forthe operating frequency, and the ferrite material loading strips haveapplied thereto a transverse DC magnetic field such that the effectivepermeability of the material is made sufficiently positive to bring theeffective dimensions of the section to the evanescent condition at thedesired frequen- It is pointed out that the magnetic field H for thefilters of FIGS. 8, 11, 12 and 14 are applied in the same manner as forthe filter of FIGS. 1 and 9. That is, by an electromagnet such as shownin FIG. 1 or by a permanent magnet 23 such as shown in FIG. 9.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:

1 An I-I-band-pass filter or filter section comprising:

a first section of waveguide;

ferrite-loading means mounted within said first section waveguide;

means for applying a transverse unidirectional magnetic field tosaidferrite-loading means for changing the effective dimensions of saidsection of waveguide with respect to the propagating wave so as to varythe cutoff frequency above the operating passband frequencies; and

means for terminating said section of waveguide including a capacitivescrew having a capacitive reactance which at the center frequency of thedesired passband is the conjugate of the positive imaginarycharacteristic reactance of said first section of waveguide.

2. A band-pass filter or filter section as claimed in claim I whereinsaid ferrite-loading means is symmetrically disposed on each sidewall ofsaid first waveguide section.

3. A band-pass filter or filter section as claimed in claim 1 whereinsaid applying means includes a permanent magnet.

4. A band-pass filter or filter section as claimed in claim 1 whereinsaid applying means includes an electromagnet.

5. A band-pass filter or filter section as claimed in claim 1 whereinsaid first section of ferrite-loaded waveguide has dimensions such thatthe cutoff frequency is below the operating frequencies of said firstsection, said applied magnetic field causing the effective dimensions ofthe first waveguide section to be reduced to a value such that thecutoff frequency is above the operating frequencies of said firstsection.

6. A band-pass filter or filter section as claimed in claim 1 whereinsaid capacitive reactance means is located at the middle of the saidsection of waveguide.

7. A band-pass filter or filter section as claimed in claim 1 includinga capacitive reactance terminating means disposed at each end of thesaid section of waveguide.

field to said ferrite-loading means for changing the effectivedimensions of said section of waveguide with respect to the propagatingwave so as to vary the cutoff frequency above the operating passbandfrequencies; and

means for terminating said section of waveguide including capacitiveridge disposed in said first section of waveguide having a capacitivereactance which at the center frequency of the desired passband is theconjugate of the positive imaginary characteristic reactance of saidfirst section of waveguide.

2. A band-pass filter or filter section as claimed in claim 1 whereinsaid ferrite-loading means is symmetrically disposed on each sidewall ofsaid first waveguide section.
 3. A band-pass filter or filter section asclaimed in claim 1 wherein said applying means includes a permanentmagnet.
 4. A band-pass filter or filter section as claimed in claim 1wherein said applying means includes an electromagnet.
 5. A band-passfilter or filter section as claimed in claim 1 wherein said firstsection of ferrite-loaded waveguide has dimensions such that the cutofffrequency is below the operating frequencies of said first section, saidapplied magnetic field causing the effective dimensions of the firstwaveguide section to be reduced to a value such that the cutofffrequency is above the operating frequencies of said first section.
 6. Aband-pass filter or filter section as claimed in claim 1 wherein saidcapacitive reactance means is located at the middle of the said sectionof waveguide.
 7. A band-pass filter or filter section as claimed inclaim 1 including a capacitive reactance terminating means disposed ateach end of the said section of waveguide.
 8. A band-pass filter orfilter section as claimed in claim 1 further comprising: first andsecond and third sections of waveguide in which a propagated mode canexist at the operating frequency of the system; and means coupling saidfirst section of waveguide between said second and third lengths ofwaveguide. 9 An H-band-pass filter or filter section comprising: a firstsection of waveguide; ferrite-loading means mounted within said firstsection waveguide; means for applying a transverse unidirectionalmagnetic field to said ferrite-loading means for changing the effectivedimensions of said section of waveguide with respect to the propagatingwave so as to vary the cutoff frequency above the operating passbandfrequencies; and means for terminating said section of waveguideincluding capacitive ridge disposed in said first section of waveguidehaving a capacitive reactance which at the center frequency of thedesired passband is the conjugate of the positive imaginarycharacteristic reactance of said first section of waveguide.